This invention relates generally to an apparatus and method for analyzing power quality, particularly for power delivered to a building by a utility.
Typical prior art power monitoring instruments provide inputs for signals corresponding to the voltage and current of a line to be monitored. To accommodate three-phase power systems, individual inputs are provided for each of the three phases.
Within a typical prior art power monitoring instrument, a multiplexer couples the various inputs to a single sampling circuit. Multiple cycles, typically eight, of a power signal present on a given input are sampled before the multiplexer switches to the next input. After all the inputs have been sampled, calculations must be performed on the samples before sampling can be resumed. The necessary calculations require approximately one second of computation time for single phase power and approximately two seconds for three phase power. Thus, for three phase power, over 100 cycles are missed before the instrument can resume processing the next set of sampled data. Only a small fraction of any one input signal is actually sampled. Multiplexing also introduces data skewing; a small error is introduced due to the time it takes for the multiplexer to change channels.
Prior art power monitoring instruments also record impulses. Impulses are captured and recorded by a thermal paper printer and/or disk storage device connected to the power monitoring instrument. However, thresholds must be programmed to determine the minimum magnitude of impulse to be captured and recorded. The need to program thresholds presents problems in that without advance knowledge of the characteristics of the power signal it is difficult to properly select a threshold. If a threshold is set too high, impulses of interest are missed. If a threshold is set too low, excessive recording of impulses causes the exhaustion of available paper and/or disk storage leading to the suspension of monitoring and a resulting loss of events.
Other problems are posed by the limited buffer storage available to store impulses while they are evaluated for permanent storage. Normal, expected impulses in a power signal can block the recording of aberration impulses that are desired to be recorded. For example, full wave rectifiers incorporated in typical power supply transformers introduce two charging impulses per cycle. With three phase power, six impulses are introduced every cycle. However, the available buffer storage in some prior art power monitoring instruments only permits six impulses to be stored before overwriting occurs. Thus, there is no possibility of tracking impulses for longer than a single cycle and if another source of impulses exists, even one cycle cannot be thoroughly analyzed.
It is desirable to derive power quality information from samples taken over the entirety of each relevant power signal. It is also desirable to record all impulses of interest while conserving on available disk storage. What is needed is a power quality measurement apparatus which provides power quality information derived from the entirety of power signals and can store all desired power disturbance information without the need to program thresholds in advance.